In industrial sewing machines and industrial embroidering machines (hereinafter also called automatic sewing units for short), the fabric is usually held, e.g., during the sewing of pockets on shirts and pants, etc., by a fabric holder, which runs along the sewing pattern under the needle of the sewing machine corresponding to the data stored in a memory of an electronic control device.
To obtain a reproducible sewing pattern, the fabric holder and the needle must be assigned to one another in a fixed manner. To monitor and set this assignment, the fabric holder is displaced into a zero position monitored by sensors in prior-art automatic sewing units after switching on the supply voltage, and the fabric holder is then displaced from this zero position into the area of the needle of the sewing machine to carry out the sewing process. At the time of the first start-up or after repairs on the automatic sewing unit, etc., a highly accurate positioning (i.e., positioning to an accuracy of 0.1 mm) of the sensors must be performed, usually with the fabric holder in its zero position, which is associated with a time-consuming mechanical adjustment.
If the fabric holder subsequently moves to the mechanical needle zero point and the needle does not exactly fit into the positioning opening of the fabric holder, an additional adjustment of the sewing machine is performed in prior-art automatic sewing units by mechanically displacing the machine. This process is also extraordinarily time-consuming.
DE 42 34 968 C1 discloses an automatic sewing unit, in which the fabric holder is connected via at least two connecting rods of a coordinate transmission to two feed parts displaceably along a straight guide rail, such that the displacement of the feed parts brings about corresponding X/Y movements of the fabric holder. The displacement of the feed parts along the guide rails takes place by means of two drives, which can be energized by means of an electronic control device. This document offers no details concerning the accurate positioning of the fabric holder in its zero position or concerning a subsequent positioning of the fabric holder in relation to the needle (determination of the needle zero point).
DE 25 60 217 C2 discloses an automatic sewing machine with a fabric holder and a zero position circuit, which brings the fabric holder into a predetermined position in each of the two coordinate directions X and Y before a sewing process in a zero position cycle. The fabric holder is displaced by means of stepping motors.
Furthermore, DE 36 26 761 C1 discloses an arrangement comprising a sewing machine and a feed means with a fabric holder, in which a optoelectronic sensor means monitors the actual operating position.
Finally, DE 40 39 827 A1 discloses a device for the numeric-controlled setting of a tool (e.g., a fabric holder), which consists of at least two parts and whose size is variable.